碳纳米桥和空穴调制二维-共价有机骨架材料氢溢流储氢特性的理论研究

Hydrogen storage by spillover is an important issue in hydrogen economics. The 2D covalent organic frameworks (2D-COFs) are made up by organic linkers held together by condensation reactions, which have two-dimensional periodic crystalline structures. Experimental and theoretical studies have shown that the hydrogen storage of 2D-COFs by spillover is low in room temperature. The key to the application of 2D-COFs is how to improve the rate of hydrogen spillover reaction. Combining the first-principles calculations and kinetic Monte Carlo (KMC) simulations, we will study two modulating mechanisms: carbon nanobridges-modulating and hole-modulating. At first, we will design several different carbon nanobridges at the atomic and molecular scale, and systematically investigate the relationship between the constitutions of carbon nanobridges and hydrogen storage properties by spillover. The aim of this study is to explore the key factors of carbon nanobridges effects for hydrogen spillover. Second, with the halogen modification, we will study the holes effects on 2D-COFs surface morphology and configuration of electrons, and study the influence of holes on absorption thermodynamics and diffusion kinetics of hydrogen atoms. Based on the study of the reaction mechanism of hydrogen spillover in detail, we will illuminate the modification mechanism of carbon nanobridges and holes in hydrogen spillover, and find out the best way to improve the efficiency of hydrogen storage by spillover, which would provide the theoretical guidance for the practical design of hydrogen storage materials.

氢溢流储氢的研究是氢能经济发展的重要课题之一。二维-共价有机骨架材料（2D-COFs）是由一些有机基团通过缩聚反应合成的二维多孔材料。实验和理论研究显示，常温下2D-COFs的氢溢流储氢量不高。如何提高氢溢流反应的速率，是提高2D-COFs储氢量的关键。我们采用第一性原理计算结合动力学蒙特卡罗模拟的方法探索两种调制机制：碳纳米桥-调制和空穴-调制。首先，在原子与分子尺度上设计多种不同结构的碳纳米桥，研究碳纳米桥的结构与氢溢流储氢性能间的关系，探索碳纳米桥影响氢溢流储氢的关键因素。其次，采用卤族元素-修饰方法引入空穴，研究空穴对2D-COFs表面形貌、电子排布以及氢原子在2D-COFs表面吸附热力学、扩散动力学的影响。在详细研究氢溢流反应机制的基础上，揭示碳纳米桥和空穴在2D-COFs氢溢流储氢中的作用机制，获得提高2D-COFs氢溢流储氢效率的最佳途径，为设计实用的储氢材料提供理论指导。

本项目主要以共价有机骨架（COF）等二维材料为研究对象，采用第一性原理计算结合动力学蒙特卡罗（KMC）模拟的方法探索了碳纳米桥和空穴在氢溢流储氢中的作用机制，在理论上设计了高密度的储氢材料。所取得的主要研究成果如下：（1）用C24、C30富勒烯和炔烃分子在理论上构建了半球状和链状碳纳米桥模型，以COF-8为基底，分析了桥连-氢溢流作用机制。研究揭示了碳纳米桥促进氢溢流反应的原因主要有两个：①碳纳米桥表面存在裸露的碳原子，它们更易与氢原子结合；②碳纳米桥中存在的氧杂质。同时预测了在碳纳米桥修饰的COF-8表面负载5 wt% 的Pt颗粒的情况下，COF-8的储氢能力可以提高到1.8 wt% ~ 1.9 wt%；（2）通过强电负性F原子的修饰方法在COF中引入了空穴。空穴降低了Pt6团簇到COF表面的距离，同时提高了H氢原子在COF表面的化学吸附焓，从而降低的氢原子的溢流势垒和反应温度；（3）研究了氢原子在五角-石墨烯表面的吸附，预测五角-石墨烯的氢溢流储氢能力能够达到5.3 wt%，同时探索了Pt4、Pd4、Ni4、Ti4单元催化剂和Ni/Pd、Ni/Pt、Pt/Pd二元催化剂在五角-石墨烯表面的氢溢流催化效率。KMC模拟显示在二元催化剂的作用下，氢溢流的最佳温度在260 ~ 361 K之间，满足DOE对储氢材料提出的温度要求；（4）在理论上设计了Li掺杂-ILCOF-1，其最大储氢密度可达7.26 wt%，最大体积吸附量可达27.2 g/L，同时KMC模拟显示Li掺杂亚胺-COF能够在适宜的温度下储存和释放氢气，有望成为一种有效的储氢材料。本项目基本完成预期目标，部分研究成果已发表在SCI收录的学术期刊上。